Truthfully, there isn't just one kind of mountain biking, nor just one kind of road biking. Comparing statistics between two very diverse categories can produce misleading conclusions. Consider the following reasonable arguments why mountain biking is safer:
1) On a mountain bike, you're generally only avoiding stationary obstacles, while on a road bike, you're avoiding cracked-out car drivers on their cell-phones.
2) Speeds are generally lower on mountain bikes.
3) Mountain bikers are in environments where spills are inherently more likely, so they remain more vigilant and alert.
Or, consider the following reasonable arguments why road biking is safer:
1) You don't have to worry as much about road-surface obstacles, which are the dominant cause of mountain bike crashes.
2) You can choose to ride on bike paths, quiet rural roads, and other places where there is little or no danger from automobiles (the dominant danger on the road), and little or no danger from obstacles (the dominant danger on a trail).
3) Mountain biking may attract people who are willing (or enthusiastic) about accepting larger risks in order to experience greater thrills. Road cyclists, on the whole, tend to be less motivated by adrenaline.
In my own (unscientific) experience, it seems that my gung-ho mountain biker friends regularly come home from a trip nursing minor injuries. In fact, it's unusual when they come home without any story to tell about at least a near-miss. (I may, however, happen to have particularly insane friends.) My roadie friends, on the other hand, rarely seem to come home with harrowing stories or actual injuries. My conclusion is that there are likely more fatalities in road biking, but many times more injuries in mountain biking. My (unscientific) conclusion is that the over-all safety is higher for road biking, in the sense that you are less likely to get injured on any particular road trip than on any particular mountain trip.
- Warren